Vapor transport method for growing crystals

ABSTRACT

PHIDE IS GROWN. THERMODYNAMIC ANALYSIS YIELDS PERMISSIBLE RANGES FOR THE PARTIAL PRESSURES OF THE P2, THE GA2O, AND H2O (FROM WHICH ARE DERIVED THE INLET PRESSURES OF THE PH3 AND THE H20+H2) AND FOR THE TEMPERATURE OF THE LIQUID GA AND OF THE SUBSTRATE. UNIFORM, LARGE AREA SINGLE CRYSTALS (E.G., GAP) GROWN IN THIS MANNER CONTAIN HIGH AMOUNTS OF ELECTRICALLY ACTIVE DOPANTS (E.G., OXYGEN) AND ARE USABLE AS EFFICIENT PHOTO- AND ELECTRO-LUMINESCENT DEVICES.   SINGLE CRYSTALS OF EITHER III-V OR VI COMPOUNDS ARE DEPOSITED ON A HEATED SUBSTRATE USING A HEATED LIQUIFIED MASS OF THE METALLIC CONSTITUENT (GROUP III OR II), A HYDROGEN COMPOUND OF THE GROUP V OR VI ELEMENT AND WET HYDROGEN AS SOURCE MATERIALS WITHIN AN OPEN REACTION TUBE AT AN ELEVATED TEMPERATURE. IN A PREFERRED EMBODIMENT SINGLE CRYSTAL GAP IS GROWN USING HEATED LIQUID GALLIUM (GA), PHOSPHINE (PH3) AND WET HYDROGEN (H2O AND H2) AS SOURCE MATERIALS. THE WET HYDROGEN IS PASSED OVER THE LIQUID GALLIUM TO GENERATE A GALLIUM SUB-OXIDE (GGA2O) STREAM WHILE THE TEMPERATURE OF THE TUBE DECOMPOSES THE PHOSPHINE TO P2 AND H2 IN A STREAM. THE P2AND GA20-CONTAINING STREAMS ARE MIXED BUT THEY REACH THE HEATED SUBSTRATE WHERE THE SINGLE CRYSTAL GALLIUM PHOS-

Feb. 1.3, 1973 MAHN-JICK LIM VAPOR TRANSPORT METHOD FOR eaowmc CRYSTALS Filed Oct. 5, 1970 3 Sheets-Sheet 1 m it: TSO ZH J JI-VVE'N'T'UF? /77. (J.

LII/77 Feb. 13, 1973 MAHN-JICK UM 3,716,405

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VAPOR TRANSPORT METHOD FOR GROWING CRYSTALS United States Patent (3 3,716,405 VAPOR TRANSPORT METHOD FOR GROWING CRYSTALS Mahn-Jick Lim, Lower Makefield Township, Bucks County, Pa., assignor to Western Electric Company Incorporated, New York, N.Y.

Filed Oct. 5, 1970, Ser. No. 78,445 Int. Cl. C23c 17/02 US. Cl. 117-201 44 Claims ABSTRACT OF THE DISCLOSURE Single crystals of either III-V or II-VI compounds are deposited on a heated substrate using a heated liquified mass of the metallic constituent (Group III or II), a hydrogen compound of the Group V or VI element and wet hydrogen as source materials within an open reaction tube at an elevated temperature. In a preferred embodiment single crystal GaP is grown using heated liquid gallium (Ga), phosphine (EH and wet hydrogen (H and H as source materials. The wet hydrogen is passed over the liquid gallium to generate a gallium sub-oxide (Ga O) stream while the temperature of the tube decomposes the phosphine to P and H in a stream. The P and Ga o-containing streams are mixed but they reach the heated substrate whereat the single crystal gallium phosphide is grown. Thermodynamic analysis yields permissible ranges for the partial pressures of the P the Ga O, and H 0 (from which are derived the inlet pressures of the PH and the H O+H and for the temperature of the liquid Ga and of the substrate. Uniform, large area single crystals (e.g., GaP) grown in this manner contain high amounts of electrically active dopants (e.g., oxygen) and are usable as efiicient photoand electro-luminescent devices.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to growing or synthesizing crystals and, more specifically, both to methods of growing single crystals or synthesizing polycrystalline material by a vapor transport method and to the crystals grown by such methods.

In an even more specific sense, the present invention contemplates, inter alia, so called vapor phase epitaxy (VPE) production methods, that is, growing single crystals or crystalline material from a vapor phase of the constituents thereof onto an appropriate seed or substrate.

The present invention also contemplates nonepitaxial vapor phase production methods, that is, the synthesis of polycrystalline material from a vapor phase of the constituents thereof with or without the use of a seed or substrate.

Such crystals of either type (single crystal orpolycrystal) may be a so called III-V or -II-V compound. If the crystal is epitaxially grown (a single crystal) and is a III-V or lI-VI compound it may be semiconductive, electroluminescent and photoluminescent. The present invention is not necessarily limited, however, to the growth of potential semiconductive, electroluminescent, photoluminescent single crystals, but contemplates the growth or synthesis from the vapor phase of many other types of single crystals and polycrystals.

('2) Discussion of the prior art Crystals of many difierent types are widely used for a variety of purposes. When such crystals are single crystals, they may, with proper doping, be used to manufacture semiconductive, electroluminescent diodes and other devices such as dynodes (light amplifiers). When the crys- 3,716,405 Patented Feb. 13, 1973 tals are polycrystalline, they may be used as starting products in other processes of growing single crystals.

Because the present invention deals most specifically with semiconductive, electroluminescent, single crystals, such crystals are the primary subject of discussion herein. Polycrystals are discussed somewhat more briefly.

semiconductive, electroluminescent diodes and othe devices such as those made of gallium phosphide [GaP], gallium arsenide [GaAs], gallium arsenide-phosphide [GaAs P gallium aluminum-arsenide or silicon carbide are, when properly doped, some of the most efiicient light sources presently known.

Typically, such diodes are made of a single crystal, such as gallium phosphide, which includes a p-n junction. The crystal must be doped, typically, in the case of red lightemitting gallium phosphide with oxygen and zinc (p-type) or with tellurium, selenium or sulphur (n-type). The oxygen dopant, when properly incorporated into the crystal, is substituted at phosphorus sites, while the zinc, similarly incorporated, is substituted at gallium sites to form nearest neighbor zinc-oxygen complexes in the lattice of the crystal. Green light-emitting gallium phosphide con tains p-type material doped with nitrogen. The nitrogen dopant is isoelectronically substituted at phosphorus sites.

Forward biasing of the p-n junction injects electrons from the n-side into the p-side whereat some electrons are trapped by the zinc-oxygen complex, in the case of red light-emitting Gal. The trapped electrons subsequently trap, or recombine with, holes to produce an exiton resulting in the emission of characteristic radiant energy. Such radiant energy, in the case of the zinc-oxygen dopants in gallium phosphide, is in the red portion of the visible spectrum.

Thus, the electroluminescence of these diodes is due to the band-gap of the crystals being large enough to encompass the visible radiation spectrum. That is, electroluminescence is caused by an electron-hole recombination mechanism.

The constituents of semiconductive, electroluminescent crystals are usually compounds selected as follows:

(a) one or more elements from Column III of the Periodic Table (such as gallium)+ one or more elements from Column V or the Periodic Table (such as phosphorus or arsenic); or

(b) one or more elements from Column ll of the Periodic Table (such as cadium)|one or more elements from Column VI of the Periodic Table (such as sulphur).

Thus, in describing semiconductive, electroluminescent compounds herein there are used the terms III-V and II-VI. These two terms have acquired a rather wellestablished meaning in the 'art.

As is well known, the above-described single crystals exhibit electroluminescence only upon the proper inclusion therein of appropriate dopants. For example, when a single crystal of GaP contains substantial amounts of substitutional oxygen and zinc, it electroluminesces in the red region of the visible spectrum. The inclusion of a different dopant, for example nitrogen, results in a single crystal which may electroluminesce in the green region of the visible spectrum.

It should be noted that as used herein the terms dopant and impurity convey diametrically opposite meanings. Specifically, an impurity is any substance which, when incorporated into a crystal, affects its electrical, physical, chemical, etc., properties in some undesired fashion. A dopant is similar to an impurity but is intentionally incorporated in small amounts into the crystal to effect some desired property therein. Thus, oxygen and zinc, intentionally incorporated into a single crystal which is intended to electroluminesce in the red region of the visible spectrum, are dopants. Oxygen incorporated into a single crystal ultimately intended to electroluminesce in the green region of the visible spectrum may be an impurity.

Electroluminescent diodes and other devices made from single crystals of the IIIV or II-VI compounds are more sturdy, reliable and longer-lived than, and are accordingly replacing, conventional incandescent lamps in a number of applications. Additionally, such diodes and devices are compact, compatible with solid state circuitry and require very little power for operation.

Nevertheless, difficulties have been experienced in efiiciently and cheaply growing uniform, large-area, luminescently efiicient and smooth single crystals having sufficient quantities of appropriate dopants properly incorpo rated thereinto from which crystals such diodes and devices are made. Such efficient and cheap crystal growth is, accordingly, one object of the present invention.

Generally, three methods have been used to produce single crystals and polycrystals. In some of these methods, it is possible to incorporate appropriate dopants in the crystal so that light of a predetermined visible or other wavelength is emitted during electroluminescence. In some cases, as noted below, the proper incorporation of dopants in the crystals produced by the three prior art methods is, at present, impossible. Thus, some of the single crystals produced by prior art methods are not usable, per se, in an electroluminescent diode or other device.

First, crystals may be grown by a crystal pulling method from a stoichiometric or near-stoichiometric melt. Present crystal pulling methods have been found to be deficient for a number of reasons. Among these reasons are the necessity of high pressures (30-40 atm.), high temperatures (about 1500 C.) and facilities sufliciently elaborate to permit such high pressure and temperature; the unwanted introduction of impurities from crucibles at the pressure and temperature necessarily utilized; and the inability to consistently grow high quality crystals. Further, in many difficult-to-predict situations, the uniform incorporation of many dopants into crystals produced by the pulling method has proved difficult. Gallium phosphide single crystals grown by a liquid-encapsulationpulling method so far have not been successfully usable in electroluminescent diodes.

In a second method, which is generally deemed cumbersome, the crystals may be grown from solutions by either so called liquid phase epitaxy (LPE) or by so called solution growth. In both cases, a heated, liquid phase of the constituents of the desired crystal is made to cool by slowly lowering the temperature thereof (which effects super-saturation). Such slow lowering of the temperature encourages the growth of single crystals while discouraging the growth of polycrystals. If crystal growth is effected on a seed or substrate contained within the solution, the method is liquid phase epitaxy (LPE); when a substrate or seed is not used and the crystal growth is manifested by the random production at separated sites of single crystals within the solution, the solution growth method is being used.

An example of an LPE method (also usable for solution growth) is US. patent application Ser. No. 40,854 filed May 27, 1970, entitled Method of and Apparatus for Growing Crystal From a Solution by S. Y. Lien which is assigned to the assignee of the present invention.

It should be noted that the terms seed and substrate are used interchangeably herein. As is well known, if a crystal layer to be grown is a single crystal layer, it is necessary that the seed or substrate also be a single crystal. If polycrystal layer growth is desired, it is not necessary that the seed or substrate be a single crystal.

The third prior art method used to produce single crystals is a vapor transport epitaxial growth method.

Present vapor transport growth methods are quite simply effected, but produce single crystals having electroluminescent efficiencies much lower (at times=) than compounds grown by the second method described above.

Additionally, present vapor transport growth methods are unable to properly incorporate, in sufiicient quantities, dopants, such as oxygen, into the grown crystals. Such dopants when properly incorporated into substitutional sites of the crystal and which contribute to the photoor electroluminescence, are termed electrically active. A distinction is made herein between electrically active dopants, usually substitutional (desirable) and dopants which are merely interstitial in nature. The latter do not improve, and may degrade a crystals electroluminescent and other properties. The more electrically active dopant incorporated properly into a single crystal, the more efficient that crystal is as an electroluminescent light source. It is desirable that an efiicient vapor transport method of growing high efiiciency electroluminescent single crystals having therein proper amounts of electrically active dopants be found. Such is accordingly another object of the present invention.

The present invention is an improvement of this third type of prior art method, namely the growth of crystals from the vapor phase of the constituents thereof, and the relaization of such an improved method is accordingly an additional object of this invention.

Ignoring for the time being some of the shortcomings (discussed both above and below) of the third type of prior art crystal growth method, it has been found that from thermodynamic and other considerations, vapor phase growth should be more expedient than crystal pulling or solution methods. Additionally, vapor phase growth provides more convenient control over the thickness of a grown crystal layer. Specifically, vapor transport methods have been found to have the ability to produce large-area, uniform thin crystal layers, thinness being an advantage from the standpoint of the ultimate electrical use of the grown crystal as an electroluminescent light source. Thus, additional objects of this invention are to provide an improved method of vapor phase crystal growth which eliminates the short-comings of prior artmethods and which takes advantage of the inherently easier control of crystal layer thickness provided by such a method.

As noted previously, neither the prior art nor the present invention, insofar as they broadly relate to vapor phase growth, are necessarily limited to epitaxial growth of electroluminescent single crystals. Rather, the broader scope of, and a primary object of this invention, is the growth, both epitaxial and non-epitaxial, of crystals, both single crystals and polycrystals, from the vapor phase in an efiicient manner which effects grown crystals having improved properties.

There are two generic types of prior art vapor transport methods presently used to grow crystals. The first method is a halide transport method; the second method involves the decomposition, in the presence of water vapor, of a polycrystalline compound into its constituents (in a vapor phase) followed by the transport and deposition of the constituents onto a seed or substrate.

In the first or halide transport method, halides of the crystals constituents, for example a Group III element such as gallium and a Group V element such as phosphorus, are transported in a heated state to a cooler substrate or seed whereat crystalline growth occurs. Where such a method is practiced, there is typically grown a crystal compound, such as gallium phosphide having a light-emitting efficiency approaching zero. Such lack of light-emitting efiiciency is due to the fact that, in the halide transport method, some dopants (e.g., oxygen) apparently cannot be properly incorporated into the grown crystal. As a practical matter, single crystals grown by the halide transport method have been primarily used for substrate or seed materials in LPE methods.

Accordingly, another object of the present invention is to provide a novel method of growing efficient electroluminescent single crystals by methods as simple as, but more expedient than, prior art halide transport methods.

In the second transport method dry hydrogen passes over a dopant source (e.g., zinc) located in an open heated reaction tube, if it is desired to grow an electroluminescent compound. The wet hydrogen first passes over a heated source of a polycrystalline compound of the constituents of the crystal it is desired to grow. For example, this heated source may be a heated mass of polycrystalline gallium phosphide. As the wet hydrogen stream passes over the heated constituent source, the polycrystalline phase volatilizes and is entrained in the stream. The wet hydrogen, now combines with the dry hydrogen, containing portions of the dopant source and the mixture impinges on the substrate or seed where, due to a maintained temperature difference, a single crystal layer (appropriately doped) grows. In this process, a specific temperature profile must be maintained within the furnace. Specifically, the constituent source must be maintained at the high temperature end of the temperature profile (about 1100 C.). The substrate must be maintained at a lower temperature. The temperature gradient so produced drives the chemical system into nonequilibrium whereat crystals are grown. It is absolutely necessary that such a temperature gradient be maintained.

Considering the other advantagesof vapor phase epitaxy (VPE), it is a desirable end to be able to groW crystals by VPE without the necessity of this temperature profile. Moreover, it is also desirable to be able to drive the system to crystal growth by one or more variables other than temperature. Both desiderata are further objects of this invention.

This second transport method has up to the present time been capable of producing single crystals, which, when used to make electroluminescent diodes, have efiiciencies about 100 times less than crystals produced by the LPE or solution growth methods. It is believed that this low efficiency is due to the fact that the second vapor transport method cannot properly incorporate dopants (especially oxygen) into grown crystals. Accordingly, an object of the present invention is to provide an improved transport method using water vapor which permits the proper incorporation of dopants into grown crystal layers which can then be used in the production of high etficiency electroluminescent diodes.

Generally speaking, the second transport method is marked by a lack of freedom in adjusting the methods parameters. The necessity of a predetermined temperature profile has been already noted. Additionally, due to the fact that the source is polycrystalline GaP, the partial pressures of gaseous Ga O and gaseous P are the same and are not independently variable. Elimination of this lack of freedom is yet a further object of this invention.

Some workers in the art have attempted to incorporate more electrically active dopants into the grown crystals by a variety of techniques. For example in oxygen-doping gallium phosphide grown by the second transport method, the partial pressure of the Water vapor or other dopant source has been raised to a point whereat increased amounts of such oxygen or other dopant are available. This expedient, however, has led to a too fast deposition rate and, consequently, to polycrystalline growth. This is believed to be due to the fact that, in prior art transport methods of the second type, the partial pressure of the gallium oxide and the partial pressure of the phosphorus are equal at equilibrium (because, of course, GaP is being decomposed). If it were possible to eliminate this constraint, itis probable that the level of oxygen or other dopant in the grown crystal could be increased while the growth rate is maintained at a level which ensures single crystalline growth. Such is, accordingly, yet another object of the present invention.

Further information concerning crystalline compounds and prior art methods of producing them may be found in the following references: Morphology of Gallium Phosphide Crystals Grown by VLS Mechanism With Gallium as Liquid-Forming Agent, by W. 'C. Ellis, C. J. Frosch and R. B. Zetterstrom in Journal of Crystal Growth, volume II, (1968), pages 61-68 (printed in the Netherlands); Visible Light From Semiconductors by Max R. Lorenz in Science, Mar. 29, 1968, volume 159- Number 3822, pages 1419-1423; Solid State Light, by A. S. Epstein and N. Holonyak in Science Journal, January 1969, pages 68-73; The Epitaxial Growth of GaP by a =Ga 'O Vapor Transport Mechanism" by C. J. Frosch in Journal of the Electrochemical Society, volume III, Number 2, February 1964, pages -184; The Reaction of GaP (s) with H O (g) and the Range of Stability of GaP (s) under Pressure of Ga O and P by C. D. Thurmond and C. J. Frosch in Journal of Electrochemical Society, volume III-Number 2, February 1964, page 184; The Preparation and Properties of Vapor-Deposited Epitaxial GaAs P Using Arsine and Phosphine, by J. J. Tietjen and J. A. Amick in Journal of Electrochemical Society, volume 113 -Number 7, July 1966, pages 724-728; and the following US. lPats. 3,406,048; 3,462,320; and 3,496,429.

It should be pointed out that only VPE methods have demonstrated the ability of producing on a continuous basis p-type and then n-type (or vice versa) electroluminescent compounds. In other crystal growth methods one or the other conductivity type must generally be grown, followed by the placing of that type in another system where the other type is then grown thereon. Consequently, a further object of this invention is the provision of a method having the ability to produce, on a continuous basis, p-type and n-type compounds to form a p-n junction;

SUMMARY OF THE INVENTION With the above-mentioned and other objects in view, the present invention contemplates a new and improved method of growing single crystal or synthesizing polycrystalline materials and, more specifically, to a new and improved method of growing such crystals by a vapor phase transport method. The present invention also contemplates new and improved crystalline compounds produced by the method of this invention.

First, using thermodynamic considerations a chemical system in equilibrium is evolved which contains three factors, namely,

1) The partial pressure of a vapor species of an oxygen compound of an element selected from Groups II or III, of the Periodic Table;

(2) The partial pressure of the vapor species of an element selected from Group VI or V of the Period Table;

(3) The partial pressure of water vapor from which is produced the oxygen compound of factor (1).

The species considered in factors (1) to (3) are combeined together in a hydrogen environment at a heated substrate or seed in the gaseous phase to grow the crystal. The evolution of the equilibrium system is such that undesirable species are not present or are present in negligible amounts at the seed or substrate.

Moreover, any two of the factors (1)-(3) can be varied over a broad range of values to grow the crystals; such variation causes the third factor to assume a value which remains, like the other two factor values, within deviation limits of a hypothesized equilibrium system. In other words, in order to grow a crystal, the system must be driven into non-equilibrium. When such is done, due to the character of the system, undesirable species are not present. Non-equilibrium may be effected by holding two of the factors (1)-(3) constant and varying the third in a favorable direction.

In a preferred embodiment, the Group II or III element (factor (1)) is gallium, from Group III; the Group V or VI element (factor (2)) is phosphorus, from Group V; and the factor 3) is H O. The oxygen-containing compound of gallium is gallium sub-oxide, specifically, Ga O. Present at a subtraste or seed are the gaseous species of pages printe in t e ht From Semiconductors by etherla ax R. Lorenz in ce, Mar. 29, 19 8, v0 ume umber 3822, pages 14191423; Solid State Light, by N N M g S. Epstein and Holonyak in Science Journal, Janua y 1969, pages 68-73; The Epitaiiial Growth of GaP W [7W mmyw/llrclzmm 11 a 1, mm

1 l/l/lj i W W am y d reaction nescent co a heated onstituents ple, this heated crystalline gallium pho I t l l i i I/ et hydrogen first passes crystalline compound of 5 is desired to grow. For

y be a heated mass of e. As the wet hydrogen The transport line 43 inserted through the plug 38b has its exit end 52 positioned adjacent the substrate or seed 22 in the holder 48. As shown by the arrows 54, a gas stream emitted from the exit end 52 impinges upon the held substrate or seed 22.

A second transport line 45 has its exit end 56 positioned near a boat or dish-like container 58. As shown by the arrows 60, gases emitted from the exit end 56 of the transport line 45 impinge on the surface of a liquid mass 62. After impinging on the liquid mass 62, the gas stream passes upstream where it impinges on the held substrate or seed 22. With respect to the direction of the gas streams emitted by the line exits 52 and 56, the exit end 52 is upstream from the exit end 56. Such relative location and other standard techniques such as the maintenance of the flow within the tube by the exhaust port 40 may be ?utilized to prevent substantially all mixing of the gas streams emitted from the exits 52 and 56 prior to their concurrent arrival at the space 65 between the exit 52 of the transport tube 43 and the held substrate or seed 22. Although somewhat premature, it should be noted that while the gas streams do mix in the space 65, no reaction thereof takes place. As explained below, the chemical system within the reaction tube 34 permits reactions to occur around the held substrate 22.

A third transport line, 44 has its exit end 66 positioned near the held substrate 22. As shown by the mottled shading 67 within the space 65, gases emitted by the exit end 66 are mixed, directed and impinge on the held substrate 22. Such gases contain an appropriate dopant in the vapor phase, for incorporation into the grown crystal layer 21 (FIG. 1). Thus, the line 44 is either connected to a source of .a gaseous dopant, e.g., dimethyl zinc, or contains a boat 68 holding a mass 69 of the dopant. In the latter case, the elevated temperature of the tube 34 generates a vapor phase of the dopant mass 69 (as shown'by the cloud 70) which is entrained in a carrier gas (arrows and carried into the space 65.

A diffusion barrier 72 of any usual type, such as e.g., an alumina-plug, is located in the exit end 66 of the line 44 to prevent the gaseous species in the space 65 from entering the line 44.

The transport line 43 has its input connected to sources (not shown) of a gas mixture comprising a source of a gaseous form of the Group V or VI element to be used, e. g., phosphine and a diluent gas, such as hydrogen. Standard gas flow regulations (not shown) may be passed in series with the various sources to selectively vary the relative proportions of the gases emitted by the exit end 52.

In a preferred embodiment, where the layer 21 (FIG. 1) is to be GaP, the diluent gas is hydrogen (H the Group V or VI element is phosphorus, compounded with hydrogen as phosphine (PH After entry into the line 43 the phosphine is decomposed by the high temperature of the reaction tube 34, into phosphorus (P and hydrogen (H Thus, in the preferred embodiment, the gas stream 54 is P +H The concentration of P is, of course, selectively variable by control of the regulators. As explained later, the P (or other Group V or VI gaseous element) concentration is determined by thermodynamic considerations.

The transport line 45 has its input connected to sources (not shown) of a carrier gas and a source of water vapor capable of forming a gaseous oxide of the selected Group H or III element. Standard gas flow regulators (not shown) may be placed inseries with the various sources to selectively vary the relative proportions of the gases emitted by the exit end 56.

In the preferred embodiment where the layer 21 is GaP, the carrier gas is again hydrogen (H The H may be purified in a palladium-silver diffusion purifier (not shown) and then humidified to saturation by bubbling through a H O bath (not shown) at a constant temperature. The humidified hydrogen is then passed through a standard condenser (not shown), maintained at a constant temperature, whereby the hydrogen is saturated with water vapor.

After entry into the line 45, the H +H O mixture leaves the exit end 56 in the gas stream 60. The concentration of H 0 is, of course, selectively variable by control of the condenser temperature and the flow rate of hydrogen. As explained later, the H 0 concentration is determined by thermodynamic considerations.

The gas stream 60 impinges on the surface of the liquid mass 62 in the boat 58. Due to the temperature of the reaction tube 34, both the stream 60 and the mass 62 are heated. As the stream 60 passes over the mass 62, a reaction between the H 0 in the stream 60 and the mass 62 produces an oxide of the mass 62. In the preferred embodiment, where the crystal layer 21 is GaP, the liquid mass is gallium (Ga) and the sub-oxide thereof is Ga' O- The gas stream 64, then, contains H H 0, and Ga O (plus possible traces of other oxides, e.g., GaO). The amount of H 0 and Ga O in the stream 64 is selectively adjusted by control of the flow rate and by the temperature of the liquid gallium mass 62 and the H 0 concentration of the gas stream 60. These amounts, as explained below, are determined by thermodynamic considerations.

Present in the space 65, then, are gaseous species of H 0, Ga O, and P as well as Zn. It is assumed that the amount of Zn dopant is so low as not to affect significantly subsequent chemical reactions which grow the GaP layer 21. Specifically, proper temperature control effects growth of the GaP layer 21 by the reaction Within limits, explained below, the partial pressures of the Ga O, the P and the H 0 at the substrate 22 are independently variable. Variation of these partial pressures has an effect on the amount of electrically active oxygen in the crystal layer 21 and also an effect on the growth rate of the layer 21. It should be pointed out that, unlike the prior art where a GaP source is used, the ability to independently vary these partial pressures makes possible:

(a) Incorporation of higher degrees of electrically active oxygen in the layer 21 by increasing Ga O partial pressure;

(b) Suppression of undesirable species by maintaining the above partial pressures within thermodynamically determined stability limits; and

(c) Avoidance of polycrystalline growth (where such is not desired) by the ability to maintain the P partial pressure low.

In the prior art method using GaP as a source, a given temperature profile must be used. Specifically, the substrate must be maintained at a temperature lower than the GaP source. In the present invention use of gaseous P and Ga O as sources permits a wide range of permissible temperature profiles within the tube 34. It has been found that the only real restriction on the temperature profile 15 that it must be one which reduces the possibility bf a reaction between the gaseous species present before they reach the substrate 22.

With this in mind, the following temperature profiles have been found usable, where T is the temperature of the liquid Ga mass 62, T is the temperature in the space 65 and T is the temperature of the substrate 22.

(a) TGETSET; TS TGET; (c) T T T; s G;

(e) T T with T increasing from T to T; and (f) T T, with T decreasing from T to T.

Profiles (b)-(d) are preferred, and these profiles have proved especially effective in preventing reactions between the Ga O,- H 0, and P in the space 65 especially those reactions which deposit material from the streams 54 and 64 on the walls of the tube 34 (as well as on the exit end 52 of the line 42).

It should be pointed out that T may range from about 700 C. to at least about 1200 C., T may range from about 900 C. to about at least about 1200 C.

As previously noted the reaction at the substrate 22 is reaction (1) given above.

The 621 in the stream 64 is generated by the following reaction at the boat 58:

2 a( 2 (g)' 2 (EH- 2(5) The P is produced by the heat-effected decomposition of phosphine (PH in the line 43:

Note that the H 0 in Equation 1 (hereafter H Ol) is present at the substrate 22, while the H 0 in Equation 2 (hereafter HzQ-Z) is used toproduce the gaseous Ga O.

Within the temperature ranges given above for T and T, considering the chemical system (Ga O, P H H O) surrounding the substrate 22 in equilibrium, growth of the crystal layer 21 may be effected by driving the system in a favorable direction. Such driving may be effected by:

(a) Increasing the partial pressure of the Ga O by either raising T and/or increasing the partial pressure of H O-2 in the stream 60;

(b) Increasing the partial pressure of the P by increasing the flow rate of the PH;.,; and/or (c) Decreasing the partial pressure of H Ol at the substrate 22.

The adjustment of these parameters, as discussed below, is done against a frame of reference of a chemical system in equilibrium, so evolved that only a II VI or a III-V compound is grown. Operating within the system ensures that no undesirable species are produced, that crystals having proper amounts of electrically active dopants therein are grown, and that these crystals are smooth, large, controllably thin, dense and relatively strain-free. The crystals grown by this method also have other novel electrical, optical and metallurgical properties.

As alluded to earlier the advantages of this invention are obtainable over a broad range of parameters using the apparatus of FIG. 2 or its equivalent.

Specifically, and referring to FIG. 3, there is shown a general graph, the area within which defines a stable chemical system labelled I at the substrate 22 which defines the area of permissible growth of the layer 21. The Y- axis of the graph is the partial pressure of Ga O on a log sc'ile; the X-axis is the partial pressure of P on a log sc e.

For any given substrate temperature T, using thermodynamic considerations, line segments II, III, and IV forming the graph are derived as follows:

(A) The line segment II is a vertical line (slope=eo) having an X-intercept 100 defined by the equation:

Log of the partial pressure of P =(x /T) +7 where 11 and 112 are constants derived from-a consideration of thermodynamic functions. Based on the best currently available values for these functions:

Thus, for any temperature T at the substrate 22, the X-intercept of the vertical line II can be determined.

(B) Also bounding the area I is a line segment III. This line has a slope of The Y-intercept 101 of this line segment with the segment II is given by:

Log of the partial pressure of P =(a /T) Log of the partial pressure of Ga O=(%;)+ (7a) where a and 73 are constants derived from a consideration of thermodynamic functions. Based on the best currently available values for these functions:

a ==e--1.30 10 and Thus, for any temperature T the Y-intercept of the segment III (slope=%) with the segment II can be determined.

(C) The area I is finally bounded by a line segment IV which has a slope of -l. The X-intercept 102 of the segment IV and the segment III is given by:

Log of the partial pressure of P (on/T) Again, based on the best available thermodynamic values:

a =11.12) 10 and ,,,:5.75.

Thus, for any T, the X-intercept 102 of the point of intersection of the segments III and IV can be located.

Accordingly, for any substrate temperature T, the area I may be defined. At the temperature T and anywhere in the area 1, crystal layers 21 may be grown without the production of undesirable species which are deleterious to the efiicient operation of the crystal. Also within the area I, proper amounts of electrically active dopants are incorporated into the crystal layer 21.

It should be noted that the line segments II and IV do not terminate at the X-axis, which is arbitrarily chosen for purposes of illustration, but extend downwardly in FIG. 3 to infinity. However, such extension may place the Ga O partial pressure (Y-axis value) at very low values (less than about l 10- atrn.) whereat crystal growth is impractically slow, as is well known.

The line segments H, III, and IV bounding the area I have real chemical and thermodynamic meaning. Without going into undue detail, sufiice it to say that GaP grown within the boundaries of the enclosed area I is acceptable single crystal GaP, while GaP grown outside thereof is not. Such unacceptability is due to the fact that outside the area, improper species of the constituents of GaP are present.

Specifically, it is postulated that to the left of the line segment II, liquid gallium is precipitated; to the right of line segments III and IV either Ga O or GaPO; is precipitated.

Referring now to FIG. 4, there is shown a graph similar to FIG. 3 except that a family of contour lines 103 (dotted) have been added to represent, within the area I of desirable growth, permissible partial pressures of the H O-l, that is, the H 0 at the substrate 22 during the reaction of Equation 1. These H O lines are parallel to the line segment IV and, thus, have a slope of -1.

The Y-intercept of the H 0 contour lines 103 with the line segment II (at a given Ga O partial pressure) at any substrate temperature T may be calculated from FIG. 5. Consequently knowing the slope (=1) of the contour lines 103 and their Y-intercepts with the line segment 11 enables the entire contour line family to be drawn for any general growth area I derived as described previously.

FIG. 5 is a graph of Ga O partial pressure (log scale X-axis) versus H O partial pressure (log scale Y-axis) both at the substrate 22. To derive a line 104 representingrthe temperature T of the substrate 22, the following is performed:

First, at a given temperature T, the Ga O partial pressure is derived fromthe formula Log partial pressure of Ga O= (%;)+'y

already defined. Specifically, the Ga O partial pressures from FIG. 3 and the Ts for which they were calculated are known. Assume one such value is point 105 in FIG-5.

Using the same T, a Y-intercept in FIG. 5 is found from LOgIO partial pressure of H O=(%)+,

where, again, the constants a and W are based on thermodynamic considerations and have values of a =---'6.44 10 and Thus, the Y-intercept, such as that represented by a point 106 in FIG. 5, may be calculated. Theco-ordinate of points 105 and 106 (i.e., the point (105, 106)), is a point 107 on a line which represents T. The lines 104 are known to have a slope of +1. Thus, the point 107 and this slope, determine the line 104. The remainder of the lines 104 in FIG. 5 are generated in this same manner. Further, then, the family of H20-]. contour lines 103 in FIG. 4 may be generated at any temperature T.

Using the above considerations, the graphs of FIGS. 6 and 7 have been constructed.

- FIG. 6 is a graph similar to FIG. 4 for a T of 1050 C. of the substrate 22.

FIG. 7, shows, on the same axes, three graphs similar to FIG. 4 for temperatures T of 950 C., 1050 C., and 1100 C., respectively. The Y-intercepts 101 are found to lie on a straight line 110;the points of intersection of segments III and IV are found to be on a straight line 111. The lines 110 and 111 meet at a point 112, Where the area I is bounded only by the segments II and 'IV. The temperature T for this area I is about 1142" C. Above this temperature, the considerations, above, are still used to determine the area I, but the positions of segments IH and IV are reversed from their positions in FIGS. 4-6.

Accordingly, the partial pressures of any of the following may be independently adjusted within the stability limits shown in FIG. 4: Ga- O, H 0, and P Such independent adjustment via the apparatus of FIG. 2 within the boundaries of the area I of preferred growth of FIG. 4 permits the growth of single crystal layers 21 on the substrate 22.

To iterate, in accordance with the present invention audthe use of the apparatus of FIG. 2, the following steps are performed to grow single crystal layers 21 of GaP on the substrate 22, which layers 21 contain proper amounts of electrically active oxygen.

At a given T the area I is generated (FIG. 3).

Next, using FIG. 5, the H contour lines are constructed parallel to the segment IV (as in FIG. 4).

Lastly, any of the Ga O, P or H O partial pressures are adjusted to fall within the area I enclosed by the segments II, III, and IV. 5 Of course, no crystals are grown by a system in equilibrium in accordance with the principle of microscopic reversibility. Thus, the system must be driven into nonequilibrium to grow the crystal layers 21.

It has been found that the equilibrium system arrived at for a given T by adjusting any two of the defined variables, may be driven to grow a crystal layer 21 by the following: Y

(a) With Ga O and P partial pressures fixed, decrease the H 0 partial pressure (i.e., H201) at the substrate 22;

(b) With Ga O and H 0 partial pressures fixed, increase the P partial pressure at the substrate 22 (c) With R and H 0 partial pressures fixed, increase Ga O partial pressure at the substrate 22.

That is, at least one of these partial pressures must be deviated from equilibrium so that reaction (1) proceeds in the forward direction at a reasonable rate. Such a rate is one which avoids polycrystal growth (unless such growth is desired) and which does not produce undesirable species, such as gallium liquid, Ga O solid or GaPO solid.

Referring to FIG. 4, assume the point X represents an equilibrium system within the area I. The point X can be defined, e.g., by known values for the partial pressures of Ga O and H 0 at the substrate 22. Of course the partial pressure of P at point X (per reaction (1)) in equilibrium is determined.

By holding the Ga O and H 0 partial pressures constant, then, a favorable deviation of P form point X will grow a crystal layer 21 on the substrate 22. Such a favorable deviation is a P partial pressure greater than that a point X and less than that represented by the intersection of a line 120 parallel to the X-aXis and segment IV or III, i.e., the right-hand stability limit of area I.

Similarly, with P and H 0 fixed at point X, a favorable deviation of Ga O is between Ga O at X and Ga O at the point of intersection of a line 121 parallel to the Y- axis and line segment III or IV, i.e., the upper stability limit of area I.

Also, with P and Ga O fixed at point X, a favorable deviation of H 0 is between H O at X and H 0 less than that represented by the contour line 103 through X, again, within the area I.

As long as these adjustments lie within area I, the advantages of this invention are realized. Such adjustments are effected by procedures described earlier.

EXAMPLES The following table presents 16 examples of Ga? crystals grown in accordance with the method of the present invention. In all cases the temperature of the substrate was 1050 C.-1070 C. but tests show similar results for crystal layers 21 grown at other substrate temperatures.

All of the crystal layers 21 were single crystals, having relatively smooth surfaces, high densities (evidenced by slow etch rates), and high photoluminescence (13%) and electroluminescent efliciencies.

Thirteen of the crystal layers 21 were oxygen and zincdoped (p-type); three were undoped (n-type). Moreover, no tendency toward polycrystallinity was noted, the partial pressures of R G320, and H 0 being held to values within the meal of FIG. 6 in accordance with the invention.

P-TYPE (OXYGEN AND ZINC DOPED) GaZO pres- H2O pres- 5% PHs-lsure at sure at 1110 tem- 95% Hz, substrate substrate Zinc presperature Ha-l-Hzo input Dry Hz H2+Zn atm. 10- P2 pressure atm. 10- sure at I a at input, nput flow flow, input flow input now, except as at substrate except as substrate Example No. ccJmin era/min. ee./min. ccJmin. shown atm. 10- shown atm. 10- 1; I 20.0 300.0 25.0 75.0 25.0 1.01 1.47 6.23 2.44 20.0 300.0 25.0 75.0 25.0 9 73 10- 1.46 6.55 2.75 20.0 300.0 60.0 40.0 25.0 1.47 3.50 1.55 2.26 20.0 300.0 100.0 0.0 25.0 1.02 5.84 6.07 1.65 20.0 300.0 40 0 60.0 25.0 1.07 2.33 5.60 2.29 10.0 300.0 100.0 0.0 25.0 6 44X10- 5.84 3.12 2.00 20. 0. 300.0 50. 0 50.0 25. 0 1.14 2. 92 4. 86 2. 30 20.0 300.0 40.0 60. 0 25 0 1 11 2 33 5 19 1.85 20.0 300.0 30.0 70.0 25.0 1.10 1.75 5.26 1.84 20.0 300.0 50.0 50.0 25. 0 1. 09 2. 92 5. 36 1. 85 20. 0 300.0 60.0 40. 0 25. 0 1. 3. 50 7. 90Xl0- 3. 30 20. 0' 300.0 40. 0 60. 0 25. 0 1. 49 2. 33 1. 41 2.04 20. 0 300. 0 30. 0 70. 0 25. 0 1. 62 1. 1. 10X10- 2. 08

' N-TYPE (UNDOPED) 20.0 300.0 25.0 75.0 0.0 1.00 1.56 7.28 0.0 0. 0 300. 0 25. 0 75. 0 0. 0 2. 89X10- 1. 55 1. 64 0. 0 0. 0 0 25. 0 75. 0v 2. X10' 1. 55 1. 67 0. 0

15 At 1050 C. (at the substrate 21) crystal layers 21 of exceptional quality were produced in the following approximate ranges of values:

P partial pressure 15x10" to 6X10' atm. H O partial pressure 6X 10- to 2.3 10 atm. Ga o partial pressure 3 1-0 to 1.5 X 10* atm.

which is shown by the approximate shaded area of FIG. 6.

The growth rate ranged, in all cases, from 1 fLlTL/hfr- 50 tm./hr., and was found to be dependent on both P and Ga o partial pressure, with approximately first order dependence residing in the P partial pressure. The layers 21 ranged in thickness of up to 10 mils. on substrates 22 of %"-l" in diameter.

For the p-type layers 21 the following data was derived:

p-type carrier concentration5 X 10 cc. resistivity-.14 ohm.-crn.

Hall mobility-77 cmF/volL-sec.

Hall coefficientl1 cc./ coulomb. photoluminescent efficiency-4 It is to be understood that the above-described embodiments are simply illustrative of the principles of the invention. Various other modifications and changes may be devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. In a method of growing a gallium phosphide GaP single crystal by the vapor transport of the constituents of the crystal comprising the steps of:

maintaining a substratezin an open reaction tube at a first elevated temperature;

directing at said substrate a first gaseous stream of phosphorus P producing a second stream of a gaseous gallium suboxide 63. 0, by passing a stream of water vapor H O over a mass of liquid gallium Ga at a second elevated temperature; independently varying the partial pressures of said first gaseous stream and said second gaseous stream to increase the electroluminescent efficiency of theresultant gallium phosphide GaP crystal; and then directing said second stream at said substrate whereat said gallium suboxide Ga O and said phosphorus P react to produce the gallium phosphide GaP crystal in a layer.

2. The method of claim 1 wherein said first stream is produced by: decomposing phosphine PH said first stream comprising phosphorus P and hydrogen H 3. The method of claim 2 wherein said second tempera ture is within the range 700 6-1200 C.

4. The method of claim 3 wherein said first tempera ture is within the range 900 C.1200 C.

5. The GaP crystal grown in accordance with the meth- I d of claim 4.

6. The method of claim 4 wherein said first temperature is 1050 C.

7. The GaP crystal grown in accordance with the meth 0d of claim 6.

8. The method of claim 6 wherein said P and said Ga O areproduced independently and which includes the additional step of: mixing said first and's'econd streams and simultaneously preventing said P and said Ga O from reacting until each reaches said substrate.

9. The method of claim 8 wherein: said mixing step is effected by producing said first stream at a point intermediate said substrate and" a point whereat said second 16 bounded by the graph of FIG. 3; having an X-axis of the partial pressure of said P on a log scale and'a Y-a'xis of the partial pressure of said Ga' O on a log scale.

11. The method of claim 10 wherein:

said area I is bounded by, and said graph consists of three intersecting line segments;

a first of said segments II has a slope of co and an X-intercept at a first point;

a second of said segments IV has a slope of and a Y-intercept at its intersection with said first segment II at a second point;

a third of said segments III has a slope of -1 and an X-intercept at its point of intersection with=said second segment IV at a third point;

said first point is defined by the equation Log partial pressure of P said second point is defined by the equation Log partial pressure of Ga O and said third point is defined by the equation where T'is said elevated temperature of said substrate in degrees.

12. The method of claim 11 wherein a family of contour lines, parallel to said third segment III and having Y-intercepts with said first line segment at Ga O partial pressure values given by FIG. 4, represents the partial pressure of said H 0 within said tube.

13. The method of claim 12 where, in FIG. 4, at a given partial pressure of Ga O, as determined by a given substrate temperature T in said equation Log partial pressure of Ga O Log partial pressure of P there passes a line having a slope of +1, a. family of such lines being generated for different values ofLT.

14. The GaP crystal grown in accordance with the method of claim 13.

15. The method of claim 13 which includes the additional step of: directing at said substrate a stream containing a gaseous species of a dopant.

16. The method of claim 15 wherein said dopant is ZlIlC.

17. The GaP crystal grown in accordance with the method of claim 16.

18. A method of growing a single crystal having as constituents either (a) one or more elements selected from Group II of the Periodic Table plus one or more elements selected from Group VI of the Periodic Table or (b) one or more elements selected from Group III of the Periodic Table plus one or more elements selected from Group V of the Periodic Table, by the vapor transport of the constituents, comprising the steps of:

(a) maintaining a substrate in an open reaction tube at a first elevated temperature;

(b) directing at said substrate a first gaseous stream containing a gaseous species of the one or more Group VI element or the one or more Group V element;

(c) producing a second gaseous stream containing a gaseous species of a compound of oxygen and the Log partial pressure of H 0:

17 one or more Group H element or the one or more Group III element by passing a stream containing water vapor over a mass of the Group H or Group III element at a second elevated temperature;

(d) directing said second gaseous stream at said substrate;

(e) independently varying the partial pressures of said first gaseous stream and said second gaseous stream to improve the electroluminescent efiiciency of the resultant grown single crystal; and

(f) mixing said first and second streams and simultaneously preventing said first and second streams from reacting until they reach said substrate whereat the crystall is grown in a layer thereon by the reaction of said gaseous species of the one or more elements in step (b) and said first compound.

19. The method of claim 18 wherein:

the Group H element is a metallic element selected from the group consisting of Zn and Cd;

the Group VI element is selected from the group consisting of O, S, Se, and Te;

the Group III element is a metallic element selected from the group consisting of Al, Ga, In, and T1; and

the Group V element is N, E, As, Sb, or Bi.

20. The method of claim 19 wherein:

said preventing step is effected by producing said first gaseous stream at a point intermediate the substrate and a point whereat said second gaseous stream is produced and by maintaining a flow direction of both of said streams in the direction of the substrate from said production points.

21. The method of claim 20 wherein:

said mass is liquid gallium;

said element in said first gaseous stream is P said compound is Ga and sadi layer is single crystalline GaP, grown according to the reaction 22. The method of claim 21 wherein: the partial pressures of said H O, P and Ga O Within said reaction tube at said substrate fall within the area I bounded by the graph of FIG. 3.

23. The method of claim 22 wherein:

said first temperature is within the range 700 C.-

1200 (3., and

said second temperature is within the range 900 C.-

24. The method of claim 23 wherein: said second temperature is 1050 C.

25. The method of claim 24 wherein the partial pressures, at said substrate, of said P Ga o, and H 0 fall within the ranges 26. The GaP crystal grown in accordance with the method of claim 25.

27. A method of growing a gallium phosphide GaP crystal by the vapor transport of the constituents of the crystal comprising the steps of:

maintaining a substrate in an open reaction tube at a first elevated temperature; directing at said substrate a first gaseous stream of .phosphorus P producing a second stream of a gaseous gallium suboxide Ga O, by passing a stream of water vapor H O over a mass of liquid gallium Ga at a second elevated temperature;

directing said second stream at said substrate; and

maintaining the partial pressures of the gaseous species of H 0, P and 621 0 within said reaction tube at said substrate, whereat said gallium suboxide Ga O and said phosphorus P react to produce the gallium Log partial pressure of Ga2O= 18 phosphide GaP crystal in a layer, within the area I bounded by the graph of FIG. 3, having an X-axis of the partial pressure of said lP on a log scale and a Y-axis of the partial pressure of said Ga O on a log scale.

28. The method of claim 27 wherein:

said area I is bounded by, and said graph consists of three intersecting line segments;

a first of said segments H has a slope of co and an X-intercept at a first point;

a second of said segments IV has a slope of and a Y-intercept at its intersection with said first segment II at a second point;

a third of said segments III has a slope ,of 1 and an X-intercept at its point of intersection with said second segment IV at a third point;

said first point is defined by the equation Log partial pressure of P said second point is defined by the equation Log partial pressure Ga O= (8.69)

and said third point is defined by the equation Log partial pressure of P i, +(5.75)

where T is said elevated temperature of said substrate in degrees.

'29. The method of claim 28 wherein a family of contour lines, parallel to said third segment III and having Y-intercepts with said first line segment at Ga O partial pressure values given by FIG. 4, represents the partial pressure of said H O within said tube.

30. The method of claim 29 where, in FIG. 4, at a given partial pressure of Ga O, as determined by a given substrate temperature T in said equation and a Y-intercept therewith determined by a fourth point defined as Log partial pressure of H 0 (3.24)

there passes a line having a slope of +1, a family of such lines being generated for difierent values of T.

31. The GaP crystal grown in accordance with the method of claim 30.

32. The GaP crystal of claim 31 which is a single crystal.

3-3. The method of claim 30 which includes the addition step of: directing at said substrate a stream containing a gaseous species of a dopant.

34. The method of claim 33 wherein said dopant is zinc.

35. The GaP crystal grown in accordance with the method of claim 34.

36. The GaP crystal of claim 35 which is a single crystal. 1

37. The method of claim 27 wherein said first stream is produced by: decomposing phosphine PH said first stream comprising phosphorus 1F: and hydrogen H 38. The method of claim 37 wherein said second temperature is within the range of 7-00 C.-1200 C.

39. The method of claim 38 wherein said first temperature is within the range 900 C.1200 C.

40. The GaP crystal grown in accordance with the method of claim 39.

41. The method of claim 39 wherein said first temperature is 1050 C.

42. The GaP crystal grown in accordance with the method of claim 41.

19 43. The method of claim 41 wherein said P and said 621 0 are produced independently and which includes the additional step of: mixing said first and second streams and simultaneously preventing said P and said Ga' O from reacting until each reaches said substrate.

44. The method of claim 43 wherein: said mixing step is efiected py producing said first stream at a point intermediate said substrate and a point whereat said second stream is produced, and by maintaining the flow direction of both streams in the direction of said substrate from said production points.

References Cited UNITED STATES PATENTS 3,394,390 7/1'968- Cheney 23204 3,197,411 7/1965 Frosch 1I7-10 6 A X OTHER REFERENCES Kenneth L. Lawley: Film Making: A Delicate Job Performed Under Pressure, Electronics, November 1967.

ALFRED L. LEAVI'IT, Primary Examiner D. A. SIMMONS, Assistant Examiner US. Cl. X.R.

L 566-PT UNITED STATES PATENT OFFICE o CERTIFICATE OF CURRETION PatemNo. 3,716,1L05 Dated r r 13, 1973 lnyent It is certified that error appears in the above-identified patent and that Said Letters Patent are hereby corrected as shown below:

In the claims, column 17, claim 23, line 44, 700C should read '-900C--; line #6, "900C" slhouldr'ead TOOC-.

Signed and sealed this 16th day of April 19 74..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

